368342 Optimization of Pyrochlore Catalysts for the Dry Reforming of Methane
The dry reforming of methane (DRM) using CO2 has long been considered a viable method for converting methane from geological or biological sources into syngas, which can then be readily used in the production of a variety of chemicals and particularly liquid fuels that can more readily be shipped via pipeline.
Though DRM holds great promise, the high temperatures required for the reaction have made it very difficult to find catalysts that exhibit high activity for extended periods. Several factors often lead to the deactivation of these catalysts: the sintering of active metals, the structural rearrangement of the catalyst support causing a reduction in surface area, and the accumulation of carbon on the catalyst surface. To date, many catalyst materials have been investigated for this reaction; for example, unsupported transition metal carbides and sulfides, supported group VIII metals, and more recently perovskites and hydrotalcites have received attention. In this study, however, we have chosen to develop optimized pyrochlore catalyst materials.
Pyrochlores are crystalline oxides having high thermal stability and a general formula of A2B2O7, where A represents a rare-earth metal and B represents a transition metal. Initial experimental efforts by others showed that pyrochlores are active for DRM but the tested catalysts exhibited poor long term stability; however, more recent data suggest that this trend in deactivation may not be applicable to all pyrochlores. La2Zr2O7 (LZ) is a pyrochlore structure which has shown good long term stability, so that efforts have been made to tailor its catalytic properties, showing Rh as a promising dopant to enhance catalytic performance for DRM. In order to determine the role of Rh in the reaction performance and understand how the reaction proceeds, we are using first principles methods employing Density Functional Theory (DFT) to analyze structural stability, species adsorption, and calculate transition state energies for the reactions on this catalyst material. To date, DFT simulations are reported for pyrochlore structures but most of them deal with bulk properties of these catalysts. To our knowledge, the work done by Mantz (2011) is the only one that deals with the interactions of a species with pyrochlore surfaces.
DFT simulation results for the 2% Rh doped LZ pyrochlore have shown good agreement with experimental data for CO and CO2 adsorption. The reactivity at the catalyst surface is studied by calculating the activation energies for all elementary reaction steps. Some activation barriers are calculated using the Climbing Image Nudged Elastic Band (CI-NEB) method (computational approach), while other barriers are found by means of a Brønsted-Evans-Polanyi relationship. These data are employed in the micro-kinetic model for a batch reactor, which outputs the partial pressure profile for the gas phase species and the coverage profile for the adsorbates with respect to residence time. This model provides then a deeper understanding of the DRM reaction mechanism on the catalyst.
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